A conventional excavating apparatus for excavating a ground to form a continuous trench below a ground surface includes a lower traveling body equipped with a crawler for traveling on the ground, an upper slewing body mounted on the under traveling body, and a portal frame provided in the upper slewing body. This portal frame is provided with a pair of traverse cylinders arranged one-above-the-other, and a leader. The pair of traverse cylinders are operable to slidingly move the leader in a traverse direction parallel to the ground surface. The excavating apparatus further includes a cutter post and a chain-type cutter.
The cutter post is suspended from the leader, and the chain-type cutter is configured to be circulatingly moved while being guided by the cutter post. The chain-type cutter includes an endless chain configured to be circulatingly driven, and a plurality of bit plates arranged on the endless chain on the side of an outer periphery thereof, at intervals along a circulating movement direction of the endless chain. On each of the bit plates, a plurality of excavation bits are arranged. By moving the cutter post below the ground surface in the traverse direction while circulatingly moving the chain-type cutters, a trench is excavated in a forward movement direction of the cutter post.
JP H09-296441A discloses a technique of circulatingly moving excavation bits along corners of a cutter post. JP 2003-74084A discloses a technique for preventing sag of a chain being circulatingly moved around a cutter post disposed at an angle, wherein the cutter post is provided with a guide mechanism for restricting a position of the chain.
Recent years, as regards excavation performance of an excavating apparatus, there has been a need to increase a width of a trench to be excavated. An increase in the trench width requires increasing a width of a bit plate supporting excavation bits. In this case, due to reaction forces applied from the ground, a large rotational moment is more likely to be generated in the bit plate. As a result, a strong shear force is applied to a bolt fixedly fastening the bit plate and a chain together, leading to a problem that loosening or disengagement of the bolt occurs.